This invention relates to circuitboard assemblies and more particularly to the use of leadframes to replace power transfer traces on circuitboards as well as to provide a heat dissipation function.
It is common to mount power transistors such as MOSFETs on circuitboards having traces on one or more sides. To dissipate thermal energy from the MOSFETs, it is common to place a finned aluminum structure over the top surface of a number of MOSFETs in bonded relationship therewith to act as a common heat sink.
There are a number of disadvantages associated with the above-described structure. For one, the heat dissipating finned structure adds cost and weight to the circuitboard assembly. For another, there is often a problem with crowding due to the large number and complexity of the traces on the circuitboard.
A primary objective of the present invention is to solve the problems of the prior art structures described immediately above; i.e., to provide a structure which eliminates the need for the finned aluminum heat sink and alleviates the problems associated with trace crowding on circuitboards bearing one or more power transistors and/or equivalent devices, along with other components such as integrated circuits, capacitors, resistors and inductors.
In general, the present invention comprises the combination of a circuitboard having at least one generally planar component mounting surface and conductive traces printed thereon and a terminal structure such as a xe2x80x9cheaderxe2x80x9d mounted on the board adjacent one edge for making appropriate and practical interconnections between the traces and devices on the circuitboard and the outside world. In accordance with the invention, one or more semi-conductor power devices such as MOSFETs having planar top surfaces are mounted on the board with some electrodes connected to traces in the conventional fashion but with one or more of the high-current power electrodes connected to the terminal structure by way of a conductive metal strip, commonly known as a xe2x80x9cleadframe,xe2x80x9d most of which is elevated above the mounting surface of the circuitboard. The leadframe has a planar portion bonded to the top surface of at least one power device in thermal transfer relationship therewith to serve two important functions, i.e., to provide a conductive power connection between the power device and the header terminal and to dissipate heat from the power device during normal operation thereof.
In accordance with the preferred embodiment of the invention hereinafter described, the leadframe conductor further comprises an integral support leg which depends from an intermediate portion of the leadframe conductor into supporting engagement with the board and is typically brazed or soldered to the board in the construction or assembly process. In addition, the leadframe is formed with a plurality of upturned individual fins to add surface area which assists in heat dissipation function.
Other applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings.